A conventional rubber or plastic insulated power cable (hereinafter referred to simply as power cable) generally comprises a cable core which includes a conductor clad with an inner semiconductor layer and an insulation layer, or with an inner semiconductor layer, an insulation layer, and an outer semiconductor layer. These individual layers are formed by extruding a resin composition, which is based on an olefin resin blended with a crosslinking agent, onto the outer peripheral surface of the conductor by means of an extruder, then heating the resultant structure under pressure to decompose the crosslinking agent blended with the base resin, so that crosslinking is effected by means of the resultant radicals.
Conventionally, moreover, power cables of the 154-kV class or higher are connected by the so-called moulded joint method as follows.
First, the respective conductors of two power cables are exposed at their end portions, and those portions of inner semiconductor layers, insulation layers, and outer semiconductor layers near the exposed end portions of the conductors are cut substantially in the desired shape of a cone. Thereafter, the exposed conductors are connected to each other, and a semiconductive tape, which is made of an olefin resin composition compounded with the crosslinking agent, or a heat-shrinkable semiconductive tube made of the aforesaid resin composition is wound or put on the conductor joint and the vicinities thereof to form an inner semiconductor layer. Then, the inner semiconductor layer is wound with an insulation tape made of an olefin resin composition compounded with the crosslinking agent, or is coated with an insulating olefin resin compound blended with the crosslinking agent by extrusion into a mold, thereby forming an insulation layer. Further, this insulation layer is wound with the semiconductive tape or fitted with the heat-shrinkable semiconductive tube in the same manner as in the formation of the inner semiconductor layer, thereby forming an outer semiconductor layer. After these individual layers are formed in this manner, they are crosslinked by heating under pressure, whereby the power cables are connected together.
Conventionally, dicumyl peroxide is used as the crosslinking agent for the olefin resin, which constitutes the insulation layers and semiconductor layers of the power cables and a joint thereof.
In manufacturing the power cables, resins having higher melting points than conventional ones are tentatively used as materials for the insulation layer and semiconductor layers so that the high-temperature properties of the resultant power cables, and therefore, the reliability thereof, are improved. In particular, this improvement is in a great demand for high-voltage power cables. However, the preset temperature of the extruder should be heightened in the case where the insulation layer and semiconductor layers made of a resin with a high melting point are extruded. In this case, the following problem may arise under some conditions.
The resin composition is heated due to the shearing force of the screw of the extruder, so that its resin temperature becomes too high. Accordingly, the crosslinking agent such as dicumyl peroxide in the resin composition is partially decomposed, so that fine, amber-colored contaminants or the so-called "amber" comes about in an extruded piece. This "amber" triggers insulation breakdown, thus lowering the properties, and therefore, the reliability, of the power cables. If the "amber" is frequent, moreover, protrusions develop on the surface of the extruded piece, thereby spoiling the external appearance of the cables.
Moreover, during the manufacture of the power cable core or the manufacture of the tape for the taped moulded joint or of the insulation layer by extrusion in the extrusion moulded joint process, a meshed screen for removing contaminants is attached to the distal end portion of the extruder. The mesh size of this screen tends to be diminished in order to reduce the amount of contaminants in the insulation layer and the like, thereby improving the reliability of the power cables and the cable joint. To remove contaminants by means of the fine-mesh screen, therefore, it is necessary to pass the resin composition through the screen meshes under high pressure. In this case, however, the resin composition is heated due to a shearing force produced when it passes through the screen meshes as the resin pressure increases. As a result, the resin temperature becomes so high that the "amber" increases.
In manufacturing the power cables, therefore, the preset temperature of the extruder for the formation of the insulation layer and semiconductor layers should be set low enough to prevent "amber." This is the reason why the base resin has to be bound to an olefin resin with a relatively low melting point (about 110.degree. C. at the maximum). In order to prevent the "amber," moreover, even prevailing olefin resins require very strict preset temperature control.
The insulation layer and semiconductor layers of the power cables contain 2-phenyl-2-propanol which is a decomposition product of dicumyl peroxide for use as the crosslinking agent. This 2-phenyl-2-propanol is decomposed to produce water when the power cables are heated to 140.degree. C. or more. The produced water increases the water content of the power cables, thereby lowering the insulation breakage characteristics of the cables.
Thus, in manufacturing the power cables, the "amber" is caused by the decomposition of the crosslinking agent during the extrusion coating of the insulation layer and semiconductor layers, and the lowering of the properties and reliability of the power cables causes a critical problem. Heretofore, however, there have not been found any power cables which can enjoy high reliability without entailing the "amber" when the insulation layer and semiconductor layers are formed with use of a higher preset temperature of the extruder.